In pulse width modulated switching regulating d.c.-to-d.c. converter circuits (hereinafter referred to as PWM switching regulating d.c.-to-d.c. converter circuits or simply as PWM switching regulators) direct current at one voltage is converted to direct current at a lower voltage by applying relatively high voltage pulses to periodically charge a capacitor to the lower voltage and by applying the lower voltage, which is built-up across the capacitor, to feed direct current to the load at the lower voltage. The regulating portion of such a converter circuit is effective to regulate the duration or width of each charging pulse in such a manner that the charge stored on the capacitor will be just enough to restore the voltage across the capacitor to the pre-selected lower voltage value. In this manner, the regulated voltage which is built up across the capacitor and which is applied across the load is normally limited to a pre-selected value for certain input voltage range and a certain load resistance range.
One application of PWM switching regulating d.c.-to-d.c. converters is in a subscriber carrier telephone system in which a number of subscriber carrier units are connected at various locations to the same telephone transmission line and in which a central office d.c. power source is connected to the transmission line to supply direct current for operating the subscriber carried units. Each of the subscriber carrier units in such a system usually requires some voltage regulation and may be equipped with a PWM switching regulating d.c.-to-d.c. converter which provides a regulated voltage for operating the subscriber's transmitter and receiver carrier equipment.
A problem with PWM switching regulating d.c.-to-d.c. converters is that the regulated output voltage will objectionably rise if the input voltage exceeds a certain maximum value or if the load resistance is decreased below a certain minimum value. Both of these conditions may occur in the type of subscriber carrier system described above.
The input voltage to the PWM switching regulators in the subscriber carrier units will exceed a normal operating voltage under certain conditions in which the transmission line is not loaded to design requirements. If, for example, the number of subscriber carrier units actually connected to the transmission line is less than the number for which the carrier system is designed, line voltage drop between each subscriber carrier unit and the central office will be decreased with the result that a higher input voltage will be applied to each subscriber carrier unit.
To overcome the problem of higher-than-normal input voltages it has been the practice prior to this invention to equip the PWM switching regulator with a pre-regulator which is effective to limit the input voltage to the PWM regulator to a value which is within the regulation capabilities of the switching regulator. By limiting the input voltage to the switching regulator with a pre-regulator, the switching regulator is not exposed to excessively high input voltage and therefore may be equipped with components having a working voltage that is less than higher-than-normal input voltages to the pre-regulator.